In a primary radar system, radar signals transmitted from an antenna are reflected by remote targets such as aircraft to produce returns. A receiver connected to the antenna processes the returns to determine the range and azimuth of the target. In a secondary radar, the identity of targets such as aircraft is determined by the use of transponders on each aircraft, which are interrogated by the transmitted radar signals. For example, secondary surveillance radar (SSR) is conventionally used to provide a picture of the position and identity of all aircraft in the immediate vicinity of an airfield, typically within a radius of 100 nautical miles. A ground radar station, termed the interrogator, transmits an interrogation signal at a radio frequency of 1030 MHz, which interrogates transponders that transmit a reply to the interrogator at a radio frequency of 1090 MHz.
The interrogator transmits r.f. energy by means of an antenna with a rotating directive pattern, which is usually constituted by a mechanical rotating antenna, but which may comprise a stationary, electronically steered phased array antenna, which has broad elevation and narrow azimuth. From a knowledge of the pointing direction of the interrogator antenna directive pattern, at the time it receives a reply from an aircraft, the azimuth bearing of the aircraft is obtained. In addition, the range of the aircraft is computed based on the duration from the interrogator transmission to the reception of the resultant reply. Hence, the range and bearing of the aircraft are obtained. For a fuller review of SSR techniques, reference is directed to "Radar Systems" Paul A. Lynn, Macmillan 1987, Chapter 6, pp 109-119.
The reply provided by conventional SSR transponder on an aircraft contains a limited amount of data, normally the aircraft altitude and its local identity. Recently, it has been proposed to introduce a more extensive bi-directional high capacity digital data link between the interrogated aircraft and the interrogator, known as Mode-S, where S denotes "selective".
The internationally agreed standard technical characteristics of Secondary Surveillance Radar are defined in the International Civil Aviation Organisation (ICAO) document "Aeronautical Telecommunications, Annex 10", Section 3.8. Section 3.8.1 covers conventional SSR and Section 3.8.1 Mode-S SSR.
Mode-S SSR, according to the aforementioned standard includes two main features. The first feature allows each aircraft transponder to be given a 25 bit address which is unique world-wide. Interrogation can therefore be directed at a specific aircraft and hence be selective, and the aircraft reply also contains its unique address. The feature allows enhanced signal processing of the reply signal by the interrogator.
The second feature allows the automatic transmission of both ground to air and air to ground, of a large quantity of data usually communicated between Air Traffic Control and the pilot of the aircraft by VHF voice communication. Thus, the automatic transmission between the ground and the aircraft in a Mode-S SSR may include transmission from the ground of flight control information and local weather conditions, and transmission from the air of data concerning fuel reserves, engine performance and weather.
Additionally, the interrogator for an SSR, and in particular a Mode-S SSR, should have the capability for its transmitted power to be varied during the rotation cycle of the antenna directive pattern. The range of interrogation is a function of the power of the transmitted radar signals and so by varying the power, the system can be selective in terms of range. The power variation required is typically over a range of 12 dB down from the maximum transmitted power in increments of 2 dB. The setting accuracy at each power level is typically .+-.1 dB and the time to switch between any power level is typically in the range 10 to 50 microseconds.
Hitherto, it has been difficult to achieve an efficient, rapid switching of power for a SSR and the present invention provides a solution to this problem.